The present invention relates to connectors for mounting integrated circuit packages to circuit boards. More particularly, the present invention relates to a resilient connection for supporting an integrated circuit chip or multiple-chip module (MCM) on a circuit board, and for providing a low profile or reduced total height off the circuit board.
In the past, leadless ceramic chip carriers (LCCCs), which typically have a different thermal coefficient of expansion (TCE) than circuit boards, exhibited problems when exposed to harsh thermally cyclic environments and vibration. The solder joints, which connect the LCCC to a circuit board, often degraded and cracked over time as a result of stresses and strains induced at the solder joint by temperature cycling. Ultimately, thermal expansion and contraction in the solder joints caused both mechanical and electrical failure, resulting in failures in the electronic system.
One solution has been to provide a compliant electrical interface between the leadless chip carrier and the circuit board. Such an interface is described in detail in the U.S. Pat. No. 4,827,611 issued to Pai et al. and in the U.S. Pat. No. 5,294,039 issued to Pai et al., which are hereby fully incorporated by reference. The curved leads described in the Pai et al. patents serve as an electrically conductive path between the LCCC and the circuit board. The curved leads also form a mechanically compliant interface that withstands the damaging thermal stresses and other mechanical vibrations that degrade and destroy solder joints in previous interfaces.
Although compliant interfaces, such as the ones described in the Pai et al patents, provide an effective interface between the LCCC and the circuit board, they are subject to certain problems. The curved leads, which often come in both “S” and “C” shapes, include minimum dimensions, such as a minimum radius of curvature in the curved leads. As a result, the total height of an integrated circuit off the printed wiring board faces certain limitations when attempting to reduce the overall height of the electronics board. For example, the 0.005 inch thick copper alloy, typically used for its electrical conductivity, exhibits a minimum height when formed into leads due to the stress and fracturing of the material when bent too sharply. As such, typical “S” type leads include a minimum height profile of about 0.068 inches and typical “C” type leads include a minimum height profile of about 0.040 inches.
Likewise, other complaint leads, such as those described in the U.S. Pat. No. 6,000,126 issued to Pai et al. and the U.S. Pat. No. 6,493,238 issued to Pai et al. which are hereby fully incorporated by reference, also include certain limitations to the reduction of overall height. The U.S. Pat. No. 6,000,126 typically includes a minimum height profile of about 0.080 inches due to the handling of the pins in mass production and the need to provide sufficient compliancy for the solder joints. Further, the U.S. Pat. No. 6,493,238 typically includes a minimum height profile of about 0.030 due to the difficulties of forming copper disks from a 0.005 inch thick material, which is needed for electrical conductivity.
Unfortunately, many current electronic applications require increase packaging ability, forcing electronic designers to package more electronics into smaller volumes without sacrificing reliability and resistance to thermal cycling. For example, VersaModule Eurocard (“VME”) module applications, such as industrial, commercial and military applications, often require reduced height electronic boards. The VME specification or form factor, which has been developed by Motorola, Signetics, Mostek and Thompson CSF, includes a standard for the length, height and width of the modules to be inserted in a standard backplane. The width of the module (which can include components on both sides, board, a stiffener frame, and a heat sink) cannot exceed the envelope provided. This is also true with modem high-density, high-performance electronic modules with other form factors. As such, several LCCC components used on VME modules and backplanes require reduced height or low profile compliant leads due to total thickness restrictions.
Therefore, it would be desirable to provide a compliant lead for mounting integrated circuit packages to circuit boards which is resistant to thermal cycling and provides a low profile.